How does someone act if they're on a high dose of pain medication?

Different. I assume were talking about opioids. If that person is been accustomed to the high dose, you may see very little in the way they act. Especially if they are only taking opioids. If they were having side effects though, you would expect to see sedation, clumsiness, lack of interest, long periods of just staring. Very tiny pupils, dry mouth and sweating are also common.